1. Field of the Invention
The present invention relates generally to a bulkhead assembly for dewatering water passages of dams and, more particularly, to a floating bulkhead assembly and, most particularly, to a segmental floating bulkhead assembly, and floatable caisson for this purpose.
2. Background Information
The standard means for dewatering dam intakes and outlets, such as spillways, outlet works, penstocks and draft tubes, has been with bulkhead assemblies or stop logs placed in opposing slots set in the passageway walls. A bulkhead assembly is a one-piece fabrication that is positioned across the water passage opening in slots to allow the water passage to be dewatered without having to lower the reservoir. The bulkhead assembly is usually lowered into place from the top of the dam with a mobile crane, gantry crane or permanent hoist. For large openings, where a one-piece bulkhead assembly is impractical, a series of horizontal bulkhead assemblies, called stop logs, are placed in the slots and stacked one on top of the next, using the same type of lifting devices used for the one-piece bulkhead assemblies. Bulkhead assemblies and stop logs are made from timber, aluminum or stainless steel for small passages, but larger openings mandate steel fabrications. When not in use, the bulkhead assemblies or stop logs are suspended above the water passage or placed in a dry storage location.
The use of buoyancy for bulkhead assemblies to reduce or eliminate the need for hoists or cranes is known. Older floating bulkhead assemblies often were one-piece steel fabrications used at site-specific intakes and stored permanently in the reservoir or removed with a large capacity crane after use. These bulkhead assemblies are designed similar to a ship. The floating bulkhead assembly's bottom is filled with ballast to keep it upright, and the bulkhead assembly is partitioned into chambers that are flooded or purged to adjust the trim of the bulkhead assembly.
Many of these floating bulkhead assemblies are still in use. However, they are difficult to maneuver and operate, more costly to fabricate than conventional bulkhead assemblies, and expensive to maintain. If not maintained, floating bulkhead assemblies may be deemed unsafe to operate due to unknown conditions in the sealed chambers, internal steel corrosion or unreliable components.
Some examples of inventions concerned with bulkhead assemblies for which patents have been granted are found in the following: Mills, U.S. Pat. No. 5,634,742, and Tucker, U.S. Pat. No. 4,729,692. Additionally, various other designs have been used or considered as shown in the literature, including the Northern States Power Company and Ayres Associates hinged bulkhead assembly described in Trends, a Publication of Ayres Associates, “Dam Renovation—Hinged Floating Bulkhead Assembly Proves Flexible, Reusable”, Autumn, 1987 (“Ayres Design”), and further described by Bakken and Vonasek in Proceedings: Small Hydro 1988, Ministry of Energy, Toronto, Canada, “Floating Bulkhead Assembly Installed for Hydro Intake Repair,” July 1988, among others. However, these disclosed devices embody many of the shortcomings outlined above, resulting in a need for an economical, easily fabricated bulkhead assembly, which is readily handled without large, expensive equipment.
The Ayres Design, which utilizes wide flange steel beams, has several drawbacks compared to the use of hollow rectangular section steel tubes made from flat sheet. Fabrication using wide flange beams to create a workable caisson requires a great amount of skillful cutting and welding of the beams, which increases the cost of fabrication. Wide flange beams are not produced in many useable varieties or dimensions, and heavy customization is often required. This lack of variety also lessens the engineering options. With wide flange beams, the bottom chamber is generally required to be the sealable chamber of the caisson, which in turn, dictates or limits the engineering options for the size of the caisson. Bakken and Vonasek reference the drawbacks with the use of rolled rectangular tube sections as being quite heavy and, due to the limited depths available in rolled steel tubes, the anticipated deflections of the units at the bottom of the wall would be excessive and could potentially cause problems with the bottom seal. Also, a drawback of using large dimension tube sections, for instance, tube sections greater than approximately 0.7 meters wide, is the excess weight and cost. The device of the present invention meets these needs, while providing many additional features that are unique to the methods and structures described herein.
FIG. 13A and FIG. 13B is a depiction of a prior art needle-beam cofferdam 400. Details of cofferdam 400 were presented in part by applicant at Waterpower '95, San Francisco, Calif., July 1995, reprinted from WATERPOWER '95 Proceedings of the International Conference on Hydropower, Published by ASCE under the title “Closure Methods for Large Intakes” by Frederick Lux II, P. E., Richard M. Rudolph, P. E., and Richard K. Frithiof, P. E., which is hereby incorporated herein by reference.
Cofferdam 400 consist of one or more horizontal structural members that act as support beams for vertical wood or steel members (needles). Generally, such closure method is used for intakes with shallow depths and long spans. For reservoir heads less than about 2.5 meters high, wood needles can be used with a single support beam at the top and a seat at the crest. However, interlocking steel sheet-pile needles are usually the most economical and provide adequate sealing for most intakes.
Alternative needle-beam or similar types of arrangement applicable for other projects involves use of needle beams or of panelized systems. One such panelized system includes that presented in part by applicant at Waterpower '05, Austin Tex., July 2005, and under the publication “Reverse Needle-Beam Cofferdam for Spillway Bay Dewatering at Tom Miller Dam” by M. Leslie Boyd, P. E., Freese & Nichols, Inc.; Frederick Lux III, P. E., Aubian Engineering, Inc.; Gregor A. Forbes and Thomas M. Glynn, Lower Colorado River Authority, which is hereby incorporated herein by reference.
The needles of such prior systems are unworkable for some applications due to the structural forces and therefore such systems are limited to relatively short length and have limited use. Such needles also are not floatable, and are not floodable in order to selectively orient needles from a generally horizontal attitude on the water to a generally vertical attitude. Applicant's present invention has overcome these and other limitations of prior designs.